There is a large number of rock bolt devices commercially available for installation within boreholes drilled into rock. These have a variety of general and special uses as rock reinforcement in both civil and mining engineering. One particular class of these devices is known as "Friction Rock Stablisers". These devices are usually compressed or expanded to fit the borehole and consequently achieve their reinforcing ability by virtue of friction (and to some extent mechanical interlock) at the interface between their outer surface and the borehole wall. These devices include the "Swellex", the "Split Set", the "Pipe Bolt" and the "Rock Nail".
"Swellex" bolts were introduced into Australia in approximately 1984. The bolt is described in Australian Patent Application no. 545968 and essentially comprises an elongated tube which has an axial depression and an internal pressure fluid receiving chamber which is closed at both ends but has a fluid inlet at one end thereof. The bolt may also comprise a fixed sleeve on one end of the tube which is the outer end of the tube, the sleeve and tube having a hole there through to communicate with the internal chamber of the tube so that the hole forms the fluid inlet. When the device is installed in an oversize bore hole and fluid is injected through the inlet the inflation pressure causes both the steel tube and to a lesser extent, the rock to expand. When the pressure is released, the rock relaxes and an interface pressure is established between the steel tube and the rock surface. Resistance to pull-out is caused by friction and mechanical interlock between the steel tube and the rough borehole wall.
A consistent and quality assured installation is the primary requirement for all rock reinforcement systems. This prerequisite is assured for the "Swellex" bolt by an elegant insertion and inflation procedure. Furthermore, this simple procedure does not require high operator expertise. However, the mechanical properties of the installed "Swellex" can be improved to address the fundamental modes of action required of rock reinforcing systems. That is, modification to the axial and shear strengths and stiffnesses.
Another form of stabilising device is the "Split Set" bolt. The "Split Set" bolt has been used in Australia since the 1970's. The Split Set bolt comprises a split tube formed from a hot-rolled steel sheet of a certain thickness which is formed in a tube rolling mill. Instead of closing the tube a longitudinal slot is left open. The split tube is cut to length, one end is tapered and a formed ring is welded to the opposite end. The tapered end allows forced insertion into an undersized borehole. The ring is intended to support a face plate at the borehole collar. In use, the "Split Set" bolt is driven into the bore hole, compressing the split tube and causing an interfacial pressure between the bolt and the rock. Resistance to pull out is due mainly to friction.
The ideal rock reinforcement device is one in which the design capacity is achieved at an appropriate stiffness without rupture of the element, irrespective of displacement. To achieve this, slip must occur between one or more of the constituent interfaces between the device and the host rock. That is, an ideal bolt may be loaded to a design load prior to slip and that a substantial proportion of this load is maintained during subsequent slippage.
The "Split Set" bolt described above goes some way towards this ideal. Slippage can occur for large displacements without rupture occurring. However, its frictional anchoring capacity is usually significantly less than its axial strength. To increase anchoring capacity a smaller bore hole may be used. However, this makes installation difficult if not impossible.
The "Swellex" bolt has the potential to achieve the stated aims of an ideal device. This could be achieved by reducing the installation pressure. However, reduction of installation pressure results in unpredictable performance. Thus, the great advantage of a consistent high quality installation is lost.